A person whose sole purpose in life is to seek out people to argue with on the internet over extremely trivial issues. Such arguments can happen on blogs, Facebook, Myspace and a host of others.
The best thing you can do to fight an internet troll is to not answer..or report them.

An Internet troll, or simply troll in Internet slang, is someone who posts controversial, inflammatory, irrelevant or off-topic messages in an online community, such as an online discussion forum or chat room, with the primary intent of provoking other users into an emotional response or to generally disrupt normal on-topic discussion.

To be a good Internet Troll you must copy and paste this definition all over the internet.

Freaking assholes who try to piss you off on places like YouTube vids and such, they're really annoying and will keep you going back and forth for HOURS!

(On Soulja Boy Tell 'Em - Bird Walk via YouTube)

SoggyNapkin: "Fucking Niggers!"
KwlKat: "Coming from someone named SoggyNapkin."
SoggyNapkin: "What does that have to do with anything, Nigger!"
(AND IT WILL GO ON AND ON AND ON AND ON UNTIL YOU JUST LOSE IT AND START CURSING BACK AT THE STUPID IDIOTIC INTERNET TROLLS)

Someone who causes dismay to someone on the internet that they don't even know, much less befrended on a site. They are most likely known to put lame or hurtful comments on someone's facebook, YouTube video (mainly videos in which the user is attempting something new like animation or clay mation), or anywhere you can openly post comments. Internet Trolls are known to also have nver contributed to the site they terrorize on and have a lame username. The best way to control the troll is to post a hurtful comment back or just delete, mark as spam, or report the comment.

Example of an Internet Troll #1
Puckerupbabii120: You need to drop dead. You guys look so retarded in this piece of shit you call a video.

Example of an Internet Troll #2
JesusSUX101: Do you really think that anyone cares about you? You gay dickhead.

An internet troll is a person who uses anonymity to cause frustration, anger, impatience or to generally be disruptive for no seemingly good reason EXCEPT to be that nuisance.

Most are souless bastards, touched by daddy/priest, and in the stead of coping with that trauma in a healthy way, take out their aggression, anger, impotence, frustration on others.

-have problems forming real-life relationships; have a hard time attracting members of the opposite/same sex,generally introverts. Though some are 'trolls-in-hiding', most are skill-less loners.

General troll behavior:disruptive forum posts; the posts are generally off-topic, or unnecessarily combative. Each contemporary popular website has its own sub-genre of troll

-can be male or female, mostly males, including the popular 'gender bender'44 yo man that acts like 14 yo girl

Female internet trolls, are far more scary then their male counterparts. Female trolls seem to better able to delude themselves into believing that their behavior is not only accepted online but that the traits that they find so well received (they are not) will transfer well to real life. usually ends badly.

EX_1...on a thread with 50 people reading/posting, the troll tries to be the first to make a comment about the picture attached; one or two people point out his/her foolishness and instead of saying he understands, is sorry, or just shutting up, the troll then proceeds to exacerbates the situation by further feigning ignorance, enraging those who do not see through the behavior. Later, when several people directly attack said troll, he responds by praising their comments, or by hitting the 'like/thumbs up' button.

EX_2:underneath an article about the impact of corporate greed in third world countries, say india, two people are having a back and forth about the article...an attention-seeking troll would pop in with something like "so how many niggers are there in india?" in an obvious attempt to illicit an emotional response; or if the article was accompanied by a family pic of a father/mother/son/daughter, a comment suck as "so how often do you think hes fucked that little girl/boy? boy he/she just LOOKS butthurt from all that fucking!", again in an obvious attempt to divert intellectual discourse, or to just project feelings of their own sex abuse, or desire to be the abuser. Just try to ignore the internet trolls.